Toner cartridges for use in copiers, printers and other image transfer devices have become the predominant source of replaceable printing toner. The cartridge typically incorporates an electrostatically or magnetically attracted toner that is usually a "one-part" system in which the colorant, fusible substrate and attractive media are all incorporated in a single particle. The developer roll is a rotating sleeve that surrounds a stationary magnet mounted concentrically within the rotating sleeve. The magnet includes North and South poles that extend axially along the magnet surface. The poles cause the toner in the tank to be magnetically attracted to the developer roll and enable its subsequent release at the "nip" between the developer roll and the image transfer drum.
U.S. Pat. No. 5,315,325 describes a technique for aligning the poles of the cylindrical magnet for optimum toner release from the developer roll to the image transfer drum. This patent is expressly incorporated herein by reference. It teaches the provision of a rotatable cylindrical magnet that is rotated to optimally align the magnet so that release of toner onto the image drum is optimized. The underlying magnet is, otherwise, unchanged and is of conventional design.
The above-described technique for adjusting a developer magnet is limited in that different magnets may have differing magnetic field properties and strengths. As a result, the print yield and print quality in different cartridges is still variable due to inherent differences between magnets. In other words, if a magnet has poor characteristics, optimization will still yield a substandard cartridge.
It is, therefore, an object of this invention to provide an improved magnet for the developer roll of a toner cartridge that reduces the inherent variability between magnets. The improved magnet should concentrate field strength where it is most needed at the nip between the developer roll and the image transfer drum. The improved magnet should be easy to produce and compatible with existing toner cartridge components. The magnet should allow the field strength to be customized for optimum performance.